In recent years, a phase-change rewritable compact disc (a CD-RW or a CD-Rewritable) or a phase-change rewritable DVD (product name: a DVD-RW or a DVD+RW; hereinafter referred to as a “RW-DVD” as appropriate) is used as a rewritable optical recording medium (such an optical recording medium will be hereinafter simply referred to as a disc or an optical disc as appropriate). The CD-RW or the RW-DVD of the phase-change type detects recorded information signals by use of a difference in reflectivity and a phase depth caused by a difference in a refractive index of a recording layer between an amorphous state and a crystalline state. Normally, the CD-RW or the RW-DVD of the phase-change type has a structure in which a lower protective layer, a phase-change recording layer (hereinafter simply referred to as a “recording layer” as appropriate), an upper protective layer, and a reflective layer are provided on a substrate. Accordingly, it is possible to control the difference in the reflectivity and the phase depth and to provide compatibility with a CD or a DVD by use of multiple interactions among these layers.
Recording on the CD-RW or the RW-DVD means overwrite recording in which a recording operation and an erasing operation are performed at the same time. Normally, in order to form one amorphous mark, a recording laser pulse is divided and a recording pulse sequence having a length corresponding to a mark length is irradiated (the divided pulse method). To be more precise, upon formation of recording mark lengths having various lengths, a laser beam to be applied on the recording layer is divided into a recording pulse for applying light having recording power Pw and a cooling pulse for applying light having low power (bias power Pb) equivalent to a retrieving power. Then, recording marks in the amorphous state having the various lengths are formed by repeatedly applying the light having the recording power Pw (the recording pulses) and the light having the bias power Pb (the cooling pulses).
FIG. 3 is a view for describing the divided pulse method in a general optical recording method. FIG. 3A shows a timing chart of a recording mark having a recording length of nT to be formed. FIG. 3B shows a timing chart of a method of dividing a recording pulse for forming the recording mark having the recording length of nT. A timing chart 200 of the recording mark having the recording length of nT shown in FIG. 3A corresponds to time duration of the recording mark having the length of nT. This timing chart 200 rises at time T1 (a starting point of an nT mark) synchronously with a reference clock, and after passage of the time period nT, falls at time T2 (an ending point of the nT mark) again synchronously with the reference clock. A timing chart 201 of the method of dividing the recording pulse for forming the recording mark having the recording length of nT shown in FIG. 3B, shows a waveform representing change with time of light energy divided into a plurality of recording pulse intervals αiT and cooling pulse intervals βiT to form the nT mark length. As shown in FIG. 3B, the recording power Pw is constant in terms of the recording pulse intervals αiT (i=integer from 1 to m), and the bias power Pb is constant in terms of the cooling pulse intervals βiT (i=integer from 1 to m). Moreover, erasing power Pe is constant in terms of intervals between the marks and in terms of intervals other than the intervals αiT (i=1 to m) and βiT (i=1 to m).
As described above, one of the reasons for dividing the laser beam to be applied on the recording layer upon formation of the recording mark lengths having the various lengths into the recording pulses for applying the light having the recording power Pw and the cooling pulses for applying the light having the bias power Pb as low as the retrieving power is to ensure a cooling rate which is necessary for forming the amorphous state. For this reason, the cooling rate becomes faster as lengths (a cooling pulse) between the pulses in the pulse sequence become longer. Meanwhile, a laser beam having the erasing power Pe lower than the recording power Pw is applied during the interval between the recording marks, whereby an amorphous mark existing prior to overwriting is crystallized.
In recent years, in order to accelerate a data transfer rate, developments of media which are recordable at a high linear velocity are in progress. Since it is necessary to crystallize (erase) the amorphous mark in a short period in overwrite recording at a high linear velocity, a recording material having a high crystallization rate is used in a recording layer. On the other hand, since the recording material having the high crystallization rate is used in the recording layer, recrystallization is apt to occur during recording the marks as well. Therefore, when using the recording material having the high crystallization rate, it is necessary to accelerate the cooling rate sufficiently in order to form the amorphous marks while suppressing recrystallization during recording the marks. For this reason, it is necessary to lengthen the cooling pulse between the recording pulses. As an effective method of lengthening the cooling pulse sufficiently, it is possible to cite a method of recording a plurality of mark lengths by use of pulse sequences having the same divided number, for example. To be more precise, there is a report concerning an optical recording method configured to record a plurality of mark lengths by use of pulse sequences having the same divided number at a linear velocity (12 m/s) which is 10 times as fast as a CD (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-331936 (see Paragraph (0178) and Paragraph (0179)).